Linear olefins are one of the most useful classes of hydrocarbons used as raw materials in the petrochemical industry and among these the linear alpha-olefins--unbranched olefins whose double bond is located at a terminus of the chain--form an important subclass. Linear alpha-olefins can be converted to linear primary alcohols by hydroformylation (oxo synthesis); alcohols of carbon number less than eleven are used in the synthesis of plasticizers whereas those of carbon number greater than eleven are used in the synthesis of detergents. Hydroformylation also can be used to prepare aldehydes as the major products which in turn can be oxidized to afford synthetic fatty acids, especially those with an odd carbon number, useful in the production of lubricants. Linear alpha-olefins also are used in the most important class of detergents for domestic use, namely, the linear alkylbenzenesulfonates, which are prepared by Friedel-Crafts reaction of benzene with linear olefins followed by sulfonation.
Another important utilization of alpha-olefins is radical hydrobromination to give primary bromoalkanes which are important intermediates in the production of thiols, amines, amine oxides and ammonium compounds. Direct sulfonation of the alpha-olefins afford the alpha-olefin sulfonates, a mixture of isomeric alkenesulfonic acids and alkanesulfones, which are effective laundry agents even in hard water and at low concentrations. Linear alpha-olefins, particularly those of eight carbons and under also are used as comonomers in the production of high density polyethylene and linear low density polyethylene.
Although linear olefins are the product of dehydrogenation of linear alkanes, the major portion of such products are the internal olefins. Preparation of alpha-olefins is based largely on oligomerization of ethylene, which has as a corollary that the alpha-olefins produced have an even number of carbon atoms. Oligomerization processes for ethylene are based mainly on organoaluminum compounds or transition metals as catalyst. Using catalytic quantities of, for example, triethylaluminum, the oligomerization of ethylene proceeds at temperatures under 200.degree. C. to afford a mixture of alpha-olefins whose carbon number follows a Schultz-Flory distribution. In the C.sub.6 -C.sub.10 range there is less than 4% branched alpha-olefins, but the degree of branching increases to about 8% as the chain length is extended to about 18. A modified process, the so-called Ethyl process, affords a high conversion of ethylene to alpha-olefins with a more controlled distribution but product quality suffers dramatically, particularly in the content of branched olefins. Thus, in the C.sub.14 -C.sub.16 range linear alpha-olefins represent only about 76% of the product.
A notable advance in the art accompanied the use of transition metals as catalysts for ethylene oligomerization. The use of, for example, nickel, cobalt, titanium, or zirconium catalysts afforded virtually 100% monoolefins with greater than 97% as alpha-olefins, under 2.5% as branched olefins, and under 2.5% as internal olefins. Since the catalysts are insoluble in hydrocarbons, oligomerization by catalyst systems based on transition metals typically is performed in a polar solvent to solubilize the catalyst. Ethylene and its oligomers have limited solubility in the polar solvents used, which permits a continuous oligomerization process, since ethylene can be introduced into the polar phase and oligomerization products can be withdrawn as the hydrocarbon phase.
Ethylene oligomerization affords alpha-olefins with a Schultz-Flory distribution which is catalyst dependent and, at least for the catalysts of major interest herein, temperature dependent to only a minor degree. A class of catalysts having a transition metal component particularly attractive as oligomerization catalysts is described in U.S. Pat. Nos. 4,689,437, 4,716,138, 4,822,915 and 4,668,8323. Using such catalysts under conditions where the Schultz-Flory distribution constant is about 0.65 affords an oligomerization product whose alpha-olefin distribution in the C.sub.8 -C.sub.16 range is particularly desirable from an economic viewpoint. That is, the economic value of ethylene oligomers may be maximized by having a Schultz-Flory distribution of about 0.65. At these operating conditions, the oligomerized reactor effluent contains oligomer compounds as well as unreacted ethylene. This unreacted ethylene must be recovered and recycled to the reaction zone. Previously, the unreacted ethylene was recovered via fractionation and then compressed before recycle to the reaction zone. Since compression is expensive, it is desirable to introduce at least a portion of recycle into the reaction zone without the necessity of compression. In accordance with the present invention, this desirable goal is achieved.
It has been discovered that the separation and recycle of supercritical ethylene can substantially reduce the amount of compression required for unconverted ethylene. Preferably, up to 90% of the ethylene can be recovered from the reactor effluent at supercritical conditions and then recycled to the reactor inlet by pumping rather than by compression. Since the supercritical separation at preferred operating conditions is not a sharp, perfect separation, about 15-25 mol % of the material recovered and recycled to the reactor are oligomers. However, one advantage to this resulting separation is that these oligomers may be used to aid in solubilizing heavy wax buildup in the reaction zone.
During the oligomerization reaction about 10% of the oligomers have 20 or more carbon atoms (C.sub.20+) which are solids at ambient temperature. The C.sub.20+ oligomers have limited solubility in the resulting hydrocarbon phase of the oligomerization process described above and therefore form a separate solid waxy phase. The oligomerization process then becomes a four-phase system; a vapor phase of ethylene, a polar solvent phase with dissolved catalyst, an immiscible liquid hydrocarbon phase and a solid phase of C.sub.20+ hydrocarbons. The formation of solids tends to plug the reactor as currently configured, so a continuous process becomes interrupted periodically due to the necessity of unplugging the reactor and even during process operation, liquid flow is impeded as solids accumulate. These solids will be a problem and prevention of solid precipitation is highly desirable. This can be effected by increasing the solubility of the heavy oligomers in the liquid hydrocarbon phase by simultaneously recycling some of the lighter oligomer fractions and recycle ethylene to the reaction zone.